There are many applications that call for a land vehicle navigation system of moderate accuracy and cost. A number of systems have been described in which the basic navigation sensor is an odometer which provides a very accurate measurement of the total distance traveled, but no directional information. Various supplementary sensors to provide directional information have been studied, ranging from a simple magnetic compass to a costly inertial heading reference system. The simplest systems can meet the cost objectives but fail to meet performance objectives; the systems that use an inertial heading reference meet performance objectives but are usually too costly for widespread applications.